This invention relates to hydrocarbon fuel processing, and more particularly to an improved shift converter and the catalysts used therein. More particularly still, the invention relates to improved catalyst compositions in, and used in, shift converters for processing hydrogen-rich gas streams, as for use in fuel cells.
Fuel cell power plants that utilize a fuel cell stack for producing electricity from a hydrocarbon fuel are well known. In order for the hydrocarbon fuel to be useful in the fuel cell stack""s operation, it must first be converted to a hydrogen-rich stream. Hydrocarbon fuels that are used by the fuel cell stack pass through a reforming process to create a process gas having an increased hydrogen content that is introduced into the fuel cell stack. The resultant process gas contains, primarily, water, hydrogen, carbon dioxide, and carbon monoxide. The process gas has about 10% carbon monoxide (CO) upon exit from the reformer.
Anode electrodes, which form part of the fuel cell stack, can be xe2x80x9cpoisoned xe2x80x9d by a high level of carbon monoxide. Thus, it is necessary to reduce the level of CO in the process gas, prior to flowing the process gas to the fuel cell stack. This is typically done by passing the process gas through a shift converter, and possibly additional reactors, such as a selective oxidizer, prior to flowing the process gas to the fuel cell stack. The shift converter also increases the yield of hydrogen in the process gas.
Shift converters for reducing the CO content of process gas are well known, and typically comprise a chamber having an inlet for entry of the process gas into the chamber, an outlet downstream of the inlet for exit of effluent from the chamber, and a catalytic reaction zone between the inlet and the outlet. The catalytic reaction zone typically contains a catalyst, or catalyst composition, for converting at least a portion of the carbon monoxide in the process gas into carbon dioxide. In operation a shift converter carries out an exothermic shift conversion reaction represented by the following equation:
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(1)
The reaction (1) between the CO and water concurrently reduces the CO content and increases the CO2 and H2 content of the process gas. The generation of additional hydrogen from this reaction is advantageous to the power plant inasmuch as hydrogen is consumed at the fuel cell anode to produce power. A discussion of one such shift converter is contained in PCT Application US97/08334 for xe2x80x9cShift Converter xe2x80x9d, published on Nov. 27, 1997 as WO 97/44123. In the shift converter of that application, a catalyst bed contains a catalyst composition of copper and zinc oxide, or copper, zinc oxide, and alumina. Such catalyst composition is further disclosed in U. S. Pat. No. 4,308,176 to Kristiansen, and has been used for a number of years to promote the shift reaction in the shift converters associated with fuel cell power plants. However, reactors using these catalyst compositions have the limitation that they must be purged with a flow of hydrogen to initially reduce them, and steps must be taken subsequent to operation to prevent significant oxidation or exposure to oxygen. In fact, the required reaction does not work, or occur, unless the catalyst is reduced. Exposure of these catalyst compositions to oxygen is, or may be, detrimental to the catalyst. This is because the catalyst is self-heating in the presence of oxygen, and it can easily heat itself to the point where catalyst particles will sinter, and thus lose surface area and decrease activity. This need to provide a reducing atmosphere and to minimize the possibility of oxygen leaks to the catalyst with a special shutdown purge and the maintenance of an inert atmosphere during shutdown, results in additional hardware and process control considerations that add to the complexity and cost of the fuel cell power plant system, particularly with regard to the shift converter.
Recent studies show that cerium oxide, or xe2x80x9cceria xe2x80x9d (CeO2), can be used in combination with a noble metal to promote the shift reaction and to eliminate the requirement that the catalyst be reduced. The combination of ceria and platinum provide a catalyst that is more oxygen tolerant than the prior catalysts. However, such ceria-promoted platinum catalysts have not demonstrated sufficient activity for the shift reaction to be useful in a reactor of a reasonable size. Rather, an unreasonably large catalyst bed would be required, particularly for mobile fuel cell power plants. Moreover, water levels typical of water-gas shift reactions may promote sintering of the ceria support.
It is thus an object of the present invention to provide a shift converter having an improved catalyst composition for efficiently converting carbon monoxide to carbon dioxide and hydrogen using a water-gas shift reaction without the need for special catalyst reconditioning.
It is a further object to provide and use an improved catalyst composition having increased activity in a shift conversion reactor for converting carbon monoxide to carbon dioxide and hydrogen using a water-gas-shift reaction without the need to protect the catalyst from exposure to air.
It is a still further object of the invention to provide and use an improved catalyst composition providing improved activity and durability over existing noble metal catalysts for the water-gas-shift reaction.
A shift converter for reducing the amount of carbon monoxide in a process gas, as for a fuel cell power plant, uses an improved catalyst composition in accordance with the invention. The shift converter includes an inlet for entry of the process gas, an outlet downstream of the inlet for exit of effluent from the chamber, and a catalytic reaction zone between the inlet and outlet. The catalyst composition of the invention resides in the catalytic reaction zone of the shift reactor and is active to convert at least a portion of the carbon monoxide and water in the process gas into carbon dioxide and hydrogen. The operation of the shift reactor with the improved catalyst composition obviates the prior requirements for pre-reducing the catalyst, providing a special post-shutdown purge, and maintaining an inert atmosphere during shutdown.
The improved catalyst composition used in the shift converter comprises a noble metal catalyst having a promoted support, which promoted support comprises a mixed metal oxide of at least cerium oxide (ceria) and zirconium oxide (zirconia). The inclusion of the zirconia with the ceria promoter increases the number of oxygen vacancies, and thus the composition""s activity. Moreover, the zirconia increases the resistance of ceria to sintering, thereby improving the durability of the catalyst composition. The mixed metal oxides, in addition to the ceria and zirconia, may include a third metal oxide, selected from the group consisting of praseodymium oxide, lanthanum oxide, neodymium oxide, and hafnium oxide, to form a ternary mix of the metal oxides. Additionally, alumina may be added to the catalyst composition, particularly if the latter is in the powder form, to improve its suitability for washcoating onto a supporting substrate.
The noble metal catalyst on the promoted support is selected from the metals of groups VIIb, VIII, and Ib of the second and third transition series of the periodic table, with platinum, palladium, rhodium, and gold being generally preferred, and platinum being particularly preferred.
The invention further includes the method of removing carbon monoxide from a process fuel gas for a fuel cell via the utilization of a shift converter which employs the improved catalyst composition.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.